Short circuits are a common and annoying problem on electronic circuit boards. Short circuits may be caused as a result of the manufacturing process of the printed wiring boards, during the process of soldering electronic components to the printed wiring boards or because of a failure within an electronic component on a printed wiring assembly.
Newly manufactured printed wiring boards often contain short circuits between traces which prevent the circuits on the circuit boards from operating properly. A trace is generally a tinned copper conductor which interconnects multiple electronic components on the circuit board to form circuits. A trace, in conjunction with other traces, forms the "artwork" on the printed wiring boards. Short circuits between traces may be the result of residual conducting material left on the circuit boards after its manufacture that inadvertently bridges two or more traces. The residual material may be so fine as to be invisible to the human eye.
Short circuits may also form during the procedure of soldering electronic components, such as chips, transistors, diodes, resistors and capacitors onto the electronic circuit board. The short circuit may be the result of solder splashes that bridge two or more traces or the result of foreign conducting material that inadvertently attaches to the circuit board.
Locating short circuits on printed wiring assemblies can be very difficult due to the circuit complexity. Short circuits may occur within the electronic components themselves. Although manufacturers of electronic components maintain high quality standards, some components may be defective with internal short circuits when they are shipped from the manufacturer's facility. Additionally, an electronic component may develop an internal short circuit after limited or prolonged use. Accordingly, it can be easy to determine that a short exists, but very difficult to locate it on a populated circuit board.
U.S. Pat. No. 5,087,874 discloses an apparatus for locating a short circuit between a first and second network with a pair of probes, a measurement device and a current source. Continually locating the probes and measuring the voltage drop between the probes provides the means for locating the short circuit. However, this process of continually comparing voltage readings may be time consuming. Furthermore, locating short circuits between circuits on electronic circuit boards populated with electronic components is extremely difficult using dual measurement probes.
U.S. Pat. No. 4,656,416 discloses a shorts locator under the control of a central processing unit. A first process identifies the second circuit to which the shorted circuit is connected. The second process locates the site of the short. A low-level AC current is injected between two shorted traces. Points along the traces are contacted by a high impedance probe. The signal is processed and the result is displayed. A known problem associated with this technique is that the injected AC source is adversely affected by the circuit reactance. For example, injecting alternating currents produces misleading indications because of the bypassing effect of various decoupling components.
It is also known to troubleshoot a large circuit board by placing it upon a grid of pointed pins, also referred to as a "bed of nails." Automatic test equipment can select various pins and inject a stimulating current or voltage. Another pin can be selected to measure the current or voltage. This technique has its drawbacks because troubleshooting a circuit board depends on nail placement.
The difficulty of locating a short circuit is compounded on circuit boards given the large number of electronic components installed on them. It is preferable to be able to troubleshoot an electronic circuit board without having to remove electronic chips and hundreds of other components from the circuit board to detect a short circuit.
Additionally, many times a technician must troubleshoot an electronic circuit board in the field with limited diagnostic resources. Some of the diagnostic equipment disclosed in the prior art is impracticable for a technician to troubleshoot circuit boards in the field.